Cook the potatoes. I did it in the pressure cooker. Cool down, peel the potatoes and crumble them with your hands.
In a saucepan, heat oil. Fry the cumin seeds and methi seeds along with the hing.
Add the chopped ginger and saute for a minute.
Blend the tomatoes and chillies till smooth. Pour into the saucepan and cook, stirring occassionally till oil floats on the top.
Add in all the spice powders and saute till the raw smell is gone.
Now add the crumbled potato, salt and sufficient water and bring the mixture to boil.
Check seasoning, add the garam masala and coriander leaves. If the sauce is too sour, add a pinch of sugar.
Switch off and serve warm with pooris.
Notes
Usually a pinch of amchur powder is added but since I didn't have it in my pantry, I didn't use it. In case you do have, you can add around 1/4 tsp along with all your spice powders.